GB No. 1 504 198 describes an impact resistant laminate comprising a number of glass sheets with a rear sheet made of polycarbonate of thickness in the range 0.76 mm to 5.6 mm having a brittle scratch-resistant coating on its exposed face, the thickness of which coating is up to 0.05 mm. The tendency of the polycarbonate sheet to spall, due to some extent to the brittle nature of the protective layer, is said to be reduced by reducing the thickness of the polycarbonate sheet to a thickness in the range 0.76 to 5.6 mm, and the results given indicate that under certain conditions of "medium power" impact, the laminate functions satisfactorily.
The use of a brittle abrasion-resistant coating on the rear polycarbonate sheet has been found to give rise to a further problem due to the incidence of minute cracks or crazing, which may extend through both the rear polycarbonate sheet and the brittle coating; perhaps due to the detrimental effect of the hard, brittle coating on stress release after laminating.
In one proposal for overcoming the tendency of the polycarbonate sheet to spall, described in GB No. 2 011 836 A, the brittle coating on the rear polycarbonate sheet has been replaced by a coating of self-healing polyurethane. In this proposal however, when the laminate includes glass, the rear polycarbonate sheet of the window structure has not been laminated to the glass but rather is glazed in a common mounting with an air space separating the laminated safety glass from the polycarbonate. In one example a multiple glazing configuration comprises two glass sheets which are laminated together, with an air space of 0.8 mm between the glass laminate and a rear unit consisting of a polycarbonate sheet 9.5 mm thick which is coated on its rear face with a 0.25 mm layer of self-healing polyurethane. The relatively thick polycarbonate sheet acts to contain spalling of the glass under impact by medium power automatic bullets.
Some abrasion resistant and self-healing polyurethanes are described in GB No. 2 070 045 A.
A further problem exists when employing an abrasion-resistant, self-healing coating to provide the rearmost, or innermost, face of the laminate, in that it is difficult to apply a thin uniform coating of self-healing material to the rear polycarbonate sheet after laminating. If the self-healing coating were to be applied to the rear polycarbonate sheet before it is laminated to one other laminae of the unit, there is a risk to the integrity of that self-healing coating.
Further it has not been possible, hitherto, to provide a curved laminated security panel with such a rearmost self-healing coating. This is because it has not proved possible to preform to a required curved configuration, a polycarbonate sheet which is impact-resistant and is to be the rear polycarbonate sheet of the laminate and which has an abrasion-resistant, self-healing coating, without risk of damage to the self-healing coating, not least during eventual stress release in the finished product.
The inventor has found that a thin polycarbonate sheet up to about 1 mm thick, and for some applications not more than about 0.64 mm thick, can act as an effective spall-resistant rear lamina in bullet resistant windows which include a rear, impact-resistant polycarbonate sheet, and which have to be capable of resisting a number of bullet impacts without spalling from the inner rear face of the laminate.
Further the inventor has provided a solution to the problem of the effective application of an abrasion-resistant, self-healing coating to such a laminate, particularly when that laminate is curved, for example for use as a bullet-resistant windshield or rear light of an automobile.
In addition the invention enables a thin polycarbonate sheet to be used as a carrier for a self-healing polyurethane coating, which thin, flexible duplex material can be applied to a curved laminate without preforming.
The invention thus provides an economical method of protecting the rear face of a flat or curved, impact-resistant laminate from abrasion and environmental degradation.